1. Field of the Invention
The present invention relates to a thin film transistor and a method for manufacturing the same, and a semiconductor device and a display device using the thin film transistor.
2. Description of the Related Art
Thin film transistors (hereinafter also referred to as “TFTs”) are already widely used in a technical field of liquid crystal displays. A TFT is a kind of field-effect transistor, and is named after the fact that a semiconductor film for forming a channel is formed thin. At present, a technique to manufacture a TFT using amorphous silicon or polycrystalline silicon for the thin semiconductor film has already been put into practical use.
A semiconductor material called “microcrystalline silicon” has been known for a long time together with amorphous silicon and polycrystalline silicon, and there also has been a report on microcrystalline silicon related to a field-effect transistor (for example, see Patent Document 1: U.S. Pat. No. 5,591,987). However, a TFT using microcrystalline silicon has been buried between an amorphous silicon transistor and a polycrystalline silicon transistor up to today; thus, there has been a delay in practical use and reports thereof are made merely at an academic society level (for example, see Non-Patent Document 1: Toshiaki Arai et al., “SID '07 DIGEST” 2007, pp. 1370-1373).
A microcrystalline silicon film can be formed over a substrate having an insulating surface, such as glass, by decomposing a source gas with plasma (weakly-ionized plasma) by a plasma CVD method; however, it has been considered that it is difficult to control generation of crystal nuclei and crystal growth because reaction proceeds in a non-equilibrium state.
Various researches have been made on microcrystalline silicon. According to a hypothesis, growth mechanism of microcrystalline silicon is as follows, first, a portion of an amorphous phase, in which atoms are aligned randomly, grows over a substrate, and then nuclei of crystals start to grow (see Non-Patent Document 2: Hiroyuki Fujiwara et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 41, 2002, pp. 2821-2828). In Non-Patent Document 2, it is considered that the density of microcrystalline silicon nuclei can be controlled with the concentration of a hydrogen gas used in forming a film because peculiar silicon-hydrogen bonds are observed on an amorphous surface when nuclei of microcrystalline silicon start to grow.
Further, influence on a growing surface of a microcrystalline silicon film due to an impurity element such as oxygen or nitrogen has also been considered. There is a finding that by reducing the concentration of the impurity element, the size of a crystal grain of a microcrystalline silicon film becomes large, and thus the defect density (especially, the defective charge density) is reduced (see Non-Patent Document 3: Toshihiro Kamei et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 37, 1998, pp. L265-L268).
Further, there is a point of view that in order to improve operation characteristics of a TFT, the purity of a microcrystalline silicon film needs to be improved, and a microcrystalline silicon film in which the concentrations of oxygen, nitrogen, and carbon are 5×1016 cm−3, 2×1018 cm−3, 1×1018 cm−3, respectively, and effective mobility is improved was reported (see Non-Patent Document 4: C. H. Lee, et al., “International Electron Devices Meeting Technical Digest (Int. Electron Devices Meeting Tech. Digest), 2006, pp 295-298). In addition, a microcrystalline semiconductor film in which a deposition temperature by a plasma CVD method is 150° C., the oxygen concentration is reduced to be 1×1016 cm−3, and effective mobility is improved was reported (see Non-Patent Document 5: Czang-Ho Lee et al., Applied Physics Letters (Appl. Phys. Lett.), Vol. 89, 2006, p 252101).